Development of efficient and versatile direct current motor drives has provided advantageous implementation in battery powered electric vehicles. A vehicle drive motor, with or without control circuitry therefor, can be incorporated in one or more wheels of a vehicle for direct provision of wheel traction. Such arrangements are exemplified in U.S. Pat. No. 6,617,746 to Maslov et al. and U.S. Pat. No. 6,492,756 to Maslov et al., both patents commonly assigned with the present invention and incorporated herein in their entirety.
FIG. 1 is a partial three dimensional perspective diagram of the structure of the central portion of a vehicle wheel in which a motor is contained, similar to the structure disclosed in the '746 patent identified above. Motor 10 comprises annular permanent magnet rotor 20 and annular stator structure 30 separated by a radial air gap. The stator comprises a plurality of ferromagnetically isolated elements. Core segments 32, made of magnetically permeable material and isolated from direct contact with each other, have respective windings 34 formed thereon. The rotor comprises a plurality of permanent magnets 22, circumferentially distributed about the air gap and affixed to an annular back plate 24, which may be of a magnetically permeable material. Each core segment is a unitary magnetic structure that comprises two poles joined in the axial direction by a linking portion, about which stator coils 34 are wound. The '756 patent illustrates stator core segments in which the poles of each pole pair are aligned in a direction perpendicular to the axis.
Within the cylindrical annular rotor structure, the stator core segments are rigidly secured by two plates 42, only one of which is illustrated in the drawing. Each plate is of a rigid circular configuration having an outer diameter and a circular cutout portion at its center that forms an inner diameter. The inner diameter is sized to fit a stationary shaft 44 and to be affixed to a member 46 integral with the shaft. Along the plate periphery, the plate is provided with holes, not shown, appropriately spaced to mate with corresponding through holes 48 in the stator core segments. Each of the plates is secured to the shaft and spaced from each other appropriately to be secured to, and to sandwich, the stator core segments at each axial side thereof via the mating holes. Suitable plate structure is illustrated in more detail in the '756 patent. Thus an annular ring is formed of stator core segments that are coextensively aligned in the axial direction across the air gap from the rotor. The stationary shaft, plates and stator structure, are contained within a housing 50, to which the annular rotor backplane and permanent magnets are attached. The housing is journalled to the shaft on the outside of the plates through appropriate bushings and bearings.
“In-wheel” motor construction raises several challenges. Considerable manufacturing complexity is involved in providing the proper structural support for maintaining the motor stator and the stationary shaft separated and positioned in relation to the wheel axle. As stator and wheel axle are produced from separate pieces, they must be manufactured with very high precision and mated together very precisely to minimize misalignments. These separate parts are often made from different materials which, due to different rates of thermal expansion, creates additional stresses during temperature cycling. The use of dissimilar metals also promotes galvanic action and accelerates corrosion. Each of these detractors increases the probability of premature motor and system failure. Provision of electrical connections to the stator elements and channeling of fluid to the stator for thermal management present additional challenges. Sleeves may be required to protect wires from sharp corners and seals may be required to contain fluid used to cool the motor.
Needs thus exist for an integrated wheel and axle structure that accommodates electrical wire access to the stator and coolant flow to the stator, that reduces stresses between the stator and the axle during thermal cycling and mechanical and electrical loading, and that provides easier service disassembly/assembly.